A healthcare network is evaluating a novel urinary biomarker assay designed to detect earl… — USMLE Step 2 CK

A healthcare network is evaluating a novel urinary biomarker assay designed to detect early-stage urothelial carcinoma. During its initial validation phase at a specialized urologic oncology referral center, the assay was tested exclusively on patients presenting with gross hematuria and a significant smoking history, a cohort where the prevalence of urothelial carcinoma is $30\%$ . In this specific high-risk setting, the assay demonstrated a sensitivity of $92\%$ and a specificity of $88\%$ . Next month, the network's administration plans to deploy this exact same assay as an annual screening tool in all asymptomatic adult patients over the age of 50 attending general internal medicine clinics, a population where the disease prevalence is estimated to be $0.4\%$ .

Which of the following best describes the expected changes in the assay's statistical parameters when applied to the general internal medicine population compared to the urologic oncology cohort?

Decreased positive predictive value, increased negative predictive value, and unchanged sensitivityCevap
B
Increased positive predictive value, decreased negative predictive value, and unchanged specificity
C
Decreased sensitivity, increased specificity, and decreased positive predictive value
D
Unchanged positive predictive value, unchanged negative predictive value, and unchanged sensitivity
E
Decreased positive predictive value, decreased negative predictive value, and unchanged sensitivity

Cevap

The positive predictive value will decrease, the negative predictive value will increase, and the sensitivity will remain unchanged.

When a diagnostic test is applied to a population with a lower disease prevalence, its intrinsic properties (sensitivity and specificity) do not change because they represent the biochemical performance of the test in given individuals. However, the predictive values are highly dependent on prevalence. As disease prevalence decreases, the number of false positives increases relative to true positives, which lowers the positive predictive value (PPV). Conversely, the overwhelming majority of the population is healthy, meaning true negatives vastly outnumber false negatives, resulting in an increased negative predictive value (NPV).

Adım Adım Çözüm

Analyze the shift in the patient population.

The disease prevalence drops drastically from

30\%

in the oncology cohort to

0.4\%

in the general screening cohort.

Identifying the change in prevalence is the critical first step to determining how test characteristics will behave.

Evaluate the impact on intrinsic test properties (Sensitivity and Specificity).

Sensitivity and specificity remain at

92\%

and

88\%

, respectively.

Sensitivity and specificity measure the test's biochemical accuracy in diseased and healthy individuals. They are intrinsic to the assay and are independent of population prevalence.

Determine the impact of decreased prevalence on Positive Predictive Value (PPV).

PPV decreases.

PPV is the probability that a positive result is a true positive. As prevalence decreases, the absolute number of true positives shrinks, while the pool of healthy people (who generate false positives) expands. Therefore, false positives overwhelm true positives, lowering PPV.

Determine the impact of decreased prevalence on Negative Predictive Value (NPV).

NPV increases.

NPV is the probability that a negative result is a true negative. In a population with

0.4\%

prevalence,

99.6\%

of patients are healthy. The massive increase in true negatives compared to the rare false negatives drives the NPV closer to

100\%

Anahtar Kavram

The effect of disease prevalence on a diagnostic test's predictive values and intrinsic properties.

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